Better at what? Ion thrusters are better at being used today, direct fusion thrusters can have much higher specific impulse, ion thrusters can be small, lightweight, and efficient, but don't provide a lot of thrust. They aren't mutually exclusive, either; you can use a fusion reactor to power an ion thruster. I don't see fusion being used for maneuvering (except when being used to generate electricity to power an ion thruster or similar engine), just as the main engine - with ion thrusters it might be possible, but they might not even have enough power for that.

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What meant by better I meant better for generic space travel in the solar system, interstellar travel, whatever. Also the Rosetta mission used ion thrusters, and it took 10 years to get to that comet. Fusion thrusters would probably take only a few months.

Ion thrusters have high exhaust velocity but don't actually put out much exhaust total, so their thrust is really low. High velocity means they don't have to carry much propellant for a given amount of delta-V, but low thrust means the mission will take a rather long time.

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Would fusion thrusters have more delta-v than ion thrusters? I can't tell because ion thruster use up fuel very slowly, but still takes 4 days just to reach 60 mph. Fusion thrusters burn fuel a little faster but provide much more thrust.

More delta-v per what? Per time, absolutely. But if they put out exhaust more slowly, they have less delta-v per quantity of propellant, and mass ratios are really what kill you on long voyages.

(Also, there's the teensy problem of we don't know how to make fusion thrusters currently.)

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I know that we currently don't have fusion thrusters already, but I'm talking about when we do. Also lets say they both have the same amount of fuel. Would the fusion thruster have more delta-v m/s than the ion thruster?

AtomicUnicorn64 wrote:I know that we currently don't have fusion thrusters already, but I'm talking about when we do. Also lets say they both have the same amount of fuel. Would the fusion thruster have more delta-v m/s than the ion thruster?

That question isn't answerable.

Higher delta-v with the same propellant mass would require higher exhaust velocity. An ion thruster can reach exhaust velocities arbitrarily close to c, just add on accelerating equipment and power requirements...not that this is an effective way to design a propulsion system. A fusion thruster could achieve higher exhaust velocity than an ion thruster of the same mass flow rate and power requirements, because the fusion produces additional power...this is the only fundamental difference, fusion rockets extract energy from fuel, ion thrusters use a separate power supply.

Fusion rockets require certain minimum conditions of plasma temperature and pressure, and some way to make effective use of the released energy, all of which will add mass to the engine and may give the fusion engine a higher minimum power requirement. On the other hand, it's conceivable for a fusion rocket to power itself, avoiding the need for a separate reactor (or for smaller craft, the mass of big solar panel wings) though that would be much more difficult than an externally-powered fusion rocket.

Plasma rockets (of which fusion rockets would be a subset) scale up to higher powers than ion engines, but are limited to lower specific impulse. Which is better depends on what you are doing: there is no such thing as "generic travel", there's a wide range of tradeoffs with respect to travel time, mass ratios, power requirements, and so on. A big delta-v budget isn't of any use if you are carrying perishable cargo that expires before you've used a fraction of it.

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Fusion is more expensive, but faster. Ion is cheaper, but slower.

First of, I would fly, and second that leads me another point. I think ion thrusters would be better for space probes, while fusion thrusters would be a lot better for manned missions.

Within the solar system, plasma/fusion rockets can scale to the thrust and power levels needed to move people in a reasonable amount of time. (for sufficiently long distances...they'd be bad for trips to the moon, and not that good for Mars)

A lot of probes might be too small for them, or just not time-sensitive enough to need them, and could get better endurance with ion drives. Probes that operate close to the sun can use solar power, so propellant that carries its own energy isn't as big a deal.

Interstellar probes need enormous amounts of delta-v, and enormous amounts of energy delivered over their operating lifetime. Fusion systems might need more propellant, but ion drives would need something with a very high energy density to power them, and fusion is very energy-dense. And if you are getting a sizable fraction of your energy from fusion, it's probably most efficient to use it directly for propulsion in the form of a rocket.